1. Field of the Invention
The present invention relates to high performance asphalt, especially paving binder asphalt described by the SUPERPAVE™ specification using the nomenclature Performance Grade PGXX-YY comprising low to moderate performance grade asphalt modified by addition of a performance improving additive to produce an asphalt boosted to the high performance grade level.
2. Description of the Related Art
Asphalt is a viscoelastic semi-solid bituminous material derived from the distillation residue of crude petroleum. Asphalt may be obtained from a variety of sources including straight run vacuum residue, mixtures of vacuum residue with diluents such as vacuum tower wash oil, paraffin distillate, aromatic and naphthenic oils and mixtures thereof, oxidized vacuum residues or oxidized mixtures of vacuum residues and diluent oils and the like. Other asphalt materials include coal tar pitch, rock asphalt and naturally occurring asphalt. Typically asphalt will have a normal atmospheric boiling point of at least about 350° C., more typically at least 440° C., and will have a penetration range from 20–500 dmm at 25° C. (ASTM D-5). Because it is hydrophobic and has good adhesive and weathering characteristics, it is widely used as a binder or cement for stone or rock aggregate in pavement construction (typically only about 5 wt % of the mixture). If, because of the nature of the crude oil distilled, the asphalt is too soft at ambient temperature, a harder product may be manufactured, for example, by increasing the vacuum distillation tower temperature, by extraction of the lighter components with liquid propane or other solvents, or by oxidizing by air blowing at an elevated temperature. However, in most cases, these manufacturing processes can produce asphalts that are too brittle at low temperature, and result in excessive cracking in cold weather. Another means of stiffening soft asphalt with less detrimental impact on its low temperature properties and so as to extend its useful temperature range is by blending with suitable polymers. Polymers can reduce the tendency of an asphalt pavement to creep and rut in warm weather by increasing its high temperature viscoelastic stiffness and at the same time permit the use of softer grades of asphalt to minimize cracking in cold weather. Thus, polymers have been added which permit the use of, in some instances, softer grades of asphalt to reduce cracking in cold weather without risking excessive softening leading to undesirable deformation/rutting in hot, summer weather. Unfortunately, many of such asphalt-polymer blends are not storage stable, especially when the polymer content exceeds 3 wt % of the mixture.
For example, a polymer such as polyethylene has been added to asphalt to produce asphalts which exhibit an increased resistance to creep or deformation. In addition, polyethylene modified asphalts must also have good compatibility between the asphalt and polyethylene, and be stable at high temperatures during storage and asphalt plant operation for ease of handling and application. Compatibility between the polyethylene and asphalt is very important to ensure that the engineering properties of both the asphalt and polyethylene are transferred to the finished product and to provide good long term performance. However, most polyethylene modified asphalts require either continuous mixing or the addition of various additives to avoid phase incompatibility (see for example U.S. Pat. Nos. 4,240,946 and 4,314,921) which require continuous high shear mixing to obtain physical dispersion of a polyolefin in bitumen, and Jew et al, Journal of Applied Polymer Science, 31, pp. 2685–2704 (1986) which discloses the addition of one or more dispersants to a polyethylene modified asphalt to avoid phase separation.
The addition of block copolymer such as styrene-butadiene-styrene (SBS) serves to modify asphalt for additional applications, such as roofing sheets, adhesives and coatings. For example, German Patent No. 2,255,173 discloses the addition of thermoplastic block copolymers (such as SBS copolymer) to straight-run asphalt after which small amounts of mineral acid (such as phosphoric acid or hydrochloric acid) are added to stabilize the resulting product, which has higher elasticity and greater range of plasticity than straight-run asphalts.
In another example, Japanese Patent No. 51-53522 discloses an asphaltic composition having improved strength due to the addition of a natural or synthetic rubber (e.g., styrene-butadiene copolymer) and an unsaturated acid, including the anhydrides of said acid. The asphaltic material may be straight-run asphalt, semi-blown asphalt, blown asphalt, tar, pitch or mixtures thereof. In addition, Japanese Patent 56-115354 teaches pre-reacting SBS polymer with unsaturated dicarboxylic acids and derivatives thereof (such as maleic acid, fumaric acid, chloromaleic acid, itaconic acid, cis-4-cyclohexene-1,2-dicarboxylic acid, endo-cis-bi-cyclo(2,2,1)-5-heptane-2,3-dicarboxylic acid, and acid anhydrides, esters, amides, imides, etc., of the dicarboxylic acid) and adding the mixture thus formed to asphalt to improve its adhesiveness, strength and deformation resistance. The resulting product is useful as a pavement structure for roadways which experience heavy repetitive loads. Among the preferred dicarboxylic acids and derivatives thereof are maleic acid, fumaric acid, and maleic anhydride.
With few exceptions, as disclosed in EP 0 885 935, the addition of polymers to asphalts has not been successful in extending the low temperature service (i.e., cracking temperature) properties below −39° C. while maintaining high temperature performance of at least 52° C.
Asphalt is made by distillation under vacuum to a cut temperature yielding, typically, a resid with particular penetration at 25° C. and viscosity at 100° C. Once it meets the specific penetration and viscosity ranges the asphalt is routed to a rundown tank where it is held for certification testing against all specifications. Once it meets the specifications the tank of asphalt is certified and sealed until shipped to the customer with no new asphalt production added. Prior to delivery it is stored in the refinery at industry typical temperatures depending on the hardness or stiffness of the asphalt. Reference is made to a best practices publication issued by the Asphalt Paving Environmental Council comprising The Asphalt Institute, the National Asphalt Paving Association and the State Asphalt Pavement Associations entitled “Best Management Practices to Minimize Emissions During HMA Construction”, EC 101 04/00. This publication contains a table insert of “Typical Asphalt Binder Temperatures”. The highest stiffness, lowest penetration asphalt listed, designated as PG 82-22 has a recommended hot mix asphalt (HMA) plant asphalt storage tank temperature range of 157 to 168° C. and a recommended HMA plant mixing temperature of 143 to 171° C. when mixing the fluid asphalt with the aggregate to adequately cover all of the aggregate surface area. In order to accommodate this industry practice the asphalt manufacturer strives to deliver to the HMA plant at an industry accepted 176° C. maximum. This requires that the asphalt manufacturer heat the asphalt at the lowest possible temperature to guarantee delivery to these temperature limits. This practice of minimizing emissions translates to mixing and blending vacuum resid asphalt and enhanced asphalts to the lowest, feasible temperature ranges. In addition to generating emissions, storage of asphalt at higher temperatures can lead to product hardening during storage and difficult handling and unloading at the HMA plant. Therefore, industry practice is to store asphalt and asphalt based products to the lower temperatures as soon as possible following manufacture.
It is not always practical to keep an asphalt-polymer blend continuously homogenized before mixing the blend with aggregates: consequently, storage stability is desirable to keep the blend from separating under storage conditions. Various methods have been suggested for making polymer modified asphaltic compositions that are sufficiently storage stable and that also have the visco-elastic properties in the ranges required for a particular application. In most of cases, this results in products for which the softening point property is strongly affected by the nature of the virgin asphalt.
Other components have been added to asphalts and polymer modified asphalts to improve their physical properties such as storage stability and viscoelastic properties. U.S. Pat. No. 5,348,994 teaches the preparation of storage stable pavement binder having improved viscoelastic properties inter alia by cosulfonation followed by neutralization of a blend of asphalt and polymer having some unsaturated bonds. U.S. Pat. No. 5,070,123 discloses a method of improving storage stability by first adding an inorganic acid such as hydrochloric acid, phosphorus pentoxide and preferably, phosphoric acid to an asphalt then adding from about 1.0 to about 15 wt % polymer to the blend. U.S. Pat. No. 4,882,373 discloses an asphaltic composition having improved tensile properties produced by the process of contacting an asphalt with a mineral acid to form an acid modified asphalt, oxidizing the resulting material to form an acid/oxygen modified asphalt, contacting it with a thermoplastic elastomer and then with specified unsaturated functional monomers. Phosphoric acid is disclosed as one of the mineral acids. U.S. Pat. No. 5,565,510 discloses the addition of polyphosphoric acid to bitumen then blending with that product a polymer containing a propylene homo- or co-polymer, a xylene soluble co-polymer fraction and optionally a xylene insoluble ethylene co-polymer fraction. U.S. Pat. No. 3,751,278 discloses treatment of an asphalt with a phosphoric acid mixture having a H3PO4 equivalent of greater than 100% concentration and including ortho-, pyro-, and poly-phosphoric acids. German patent DE 2255173 C3 teaches the use of 85% o-phosphoric acid, i.e., the addition of low molecular weight phosphoric acid.
SUPERPAVE™ criteria (as described in the June 1996 edition of the AASHTO Provisional Standards Book and 2003 revised version) can be used to define the Maximum and Minimum Pavement service temperature conditions under which the binder must perform. SUPERPAVE™ is a trademark of the Strategic Highway Research Program (SHRP) and is the term used for new binder specifications as per AASHTO MP-1 standard. Maximum Pavement Temperature (or “application” or “service” temperature) is the temperature at which the asphalt binder will resist rutting (also called Rutting Temperature). Minimum Pavement Temperature is the temperature at which the binder will resist cracking. Low temperature properties of asphalt binders were measured by Bending Beam Rheometer (BBR). According to SUPERPAVE™ criteria, the temperature at which a maximum creep stiffness (S) of 300 MPa at 60s loading time is reached, is the Limiting Stiffness Temperature, LST. Minimum Pavement Temperature at which the binder will resist cracking (also called Cracking Temperature) is equal to LST-10° C.
Certain asphalts derived from particular crude sources such as Escalante, Canadon Seco, Boscan, Menemota and Napo crudes exhibit inherently high performance and are characterized as meeting a SUPERPAVE™ binder specification of PG 58-34 to 64-34.
While such asphalts can be and typically are further modified by the addition of polymers or copolymers or by air blowing or phosphoric acid treatment, the base asphalt by being inherently a high performance grade asphalt can make do with less of such treatment or such additives or can even dispense totally with such treatment or additives.
In the case of polymer or copolymer additives the elimination of the need to employ polymers to improve and expand the range of low temperature and high temperature performance would avoid stability and separation problems. The elimination of costly additives obviously effect a significant cost saving while the elimination of air blowing reduces cost and treatment and processing complexity.
Air-blowing, for instance, employs catalysts and is used to make industrial asphalt. Air blowing equipment is costly and requires maintenance while the catalyst further increases costs and processing complexity. Further environmental concerns exist regarding air filtration requiring additional equipment and maintenance along with licensing and inspection issues. If catalysts are eliminated the use of high processing temperatures becomes necessary, on the order of 300° F. to 500° F. for 1–30 hours followed by additional heating step at about 300° F. in the presence of a polymerization catalyst, see U.S. Pat. No. 2,179,208.
U.S. Pat. No. 4,713,117 teaches a process for modifying the physical properties of bituminous materials and asphalt compositions without air blowing by mixing a particular bituminous feed material with from 0.1 to 20.0 wt % of an organic sulfonic acid, said mixing being conducted at a temperature in the range of 351° F. to 600° F. (177.2° C. to 315.5° C.) whereby the softening range of the feed is substantially increased and the penetration is significantly decreased by 5 to 80 dmm at 77° F.
The organic sulfonic acid can be an alkylbenzene sulfonic acid wherein the alkyl group contains 1–20 carbons.
U.S. Pat. No. 4,440,579 teaches a process for air blowing asphalt in the presence of organic sulfonic acids as catalyst. The organic sulfonic acid can be an alkyl benzene or alkyl naphthalene sulfonic acid wherein the alkyl group contains from 1–20 carbons. The asphalt is heated to a temperature of about 200° F. to 350° F. and thoroughly mixed with the sulfonic acid catalyst and thereafter air blown. More specifically the asphalt is heated to a temperature of from about 400–550° F. and the air blowing is for from 0.5 to about 12 hours.
It would be desirable to secure a high grade paving asphalt similar in performance to, e.g., Escalante, Canadon Seco, Menemota or Napo source asphalt and matching the performance of SUPERPAVE™ Performance Grade Binders, e.g., PG 58-34, PG 64-28, PG 64-34, etc., from low to moderate grade asphalt and without the use of air blowing or the addition of modifying polymers or copolymers.